Zinc Plated vs Hot-Dip Galvanized Fasteners

Start with the service environment

Define indoor or outdoor exposure, moisture, salt or industrial contaminants, expected service life, temperature and maintenance conditions. A coating choice should follow the environment and the joint requirement, not only a preference for a particular colour or appearance.

Zinc plating and thin zinc coatings

Zinc-plated fasteners use an electrolytic or other specified zinc-coating process. They are often considered for controlled indoor or less aggressive environments where a relatively thin coating and familiar appearance are suitable. The RFQ should still state the coating designation, thickness or corrosion-test requirement when the project controls those details.

Hot-dip galvanizing

Hot-dip galvanizing immerses the steel part in molten zinc and produces a zinc-iron alloy layer plus an outer zinc layer. The coating can affect thread fit, nut compatibility, dimensions and installation practice, so the fastener, nut and washer system should be reviewed together. The American Galvanizers Association describes the coating structure and the need to manage fit and installation for galvanized fasteners.

Questions to include in the RFQ

• Required coating process, standard and coating thickness or test method.
• Service environment and expected corrosion-protection requirement.
• Thread allowance, mating nut compatibility and installation method.
• Hydrogen embrittlement controls or post-treatment requirements where applicable.
• Inspection records, marking, packaging and protection against transit damage.